It is not certain that you should earn from cryptocurrencies only as these are earning money. The main brokering platforms allow you to go and choose both BUY and SELL options. r/Ethtrader guides you in these matters.
To be clear you can go to gain on both trends and if you feel that the coin you are interested in is down, you just need to liquidate your buy positions and buy sell positions. From r/Ethtrader you will get all the supports and suggestions regarding all these.
The strategies in question can be put into practice, however, only by resorting to the platforms that offer CFDs, it is also for this reason that they are the main and preferable channel for investing in this type of instruments.
Once you have developed your strategy, do not invest your capital immediately. In fact, the best brokers allow you to invest through a demo account, with virtual currency, before moving on to real investments.